A lot to unpack in just a few sentences lol.
No good player is turning off their aim assist. None. Zero. Every sweaty try hard player I've seen try to do that for a video, they end up struggling to get kills in whatever game they're playing. They still have the game knowledge down, movement, all the other aspects of the game, which is a big part of shooters, but then aiming they struggle. They put it back on, boom, they're beaming everyone again.
Weird argument for beginners, anyone new to FPS games is going to massively struggle with any input. A mouse might be more intuitive, because it's the better tool for aiming, but what about the other hand? I grew up on Console and switched to PC about 15 years ago, using a keyboard was not intuitive, nor was it an easy switch because of it.
What about good players with assist then? Same thing could be applied to mouse users. Not all of us can aim like Shroud, so should we get assist then? Since all the metrics for assist seemed to be balanced from the top down?
Modern players using a controller look like MnK players. There's times it takes me a minute or two to figure out if a decent player is on a mouse or controller while spectating, or watching a clip. A good player, it's nearly indistinguishable at times. They can do all the same movement a MnK player can do. Being able to move a mouse quickly doesn't make you more accurate, in fact, it's usually the very opposite. If I'm off by a single pixel on a mouse, I miss. If I don't track perfectly, the aim assisted controller player will always miss less shots.
I'm not sure what the numbers are for Console versus PC, I'm not sure it's vastly different, but that's also due to a few factors because of Dice dropping the ball on the PC market. It could gain popularity on PC again, and it should be in the top 10 spots of Steam effortlessly. I made that a point in my OP, PC players don't need crossplay. Giving controller players assist encourages more people to use them over MnK because it's now a lot easier to be consistent.
I could write a literal novel on why this whole thing is absolutely ridiculous, but controller players will never concede on any point. That's why the real solution is separating Console from PC players, at least by default. If Portal is going to be a thing, then maybe it should be an option to join each other, but not by default.